The present invention relates to an electrophotographic photoreceptor with a photoconductive layer formed of amorphous silicon (a-Si).
Conventionally, a photoreceptor employing amorphous Se or amorphous Se doped with impurities such as As, Te and Sb, or ZnO or CdS dispersed in the resin binder is in use as an electrophotographic photoreceptor. However, those photoreceptors still present some problems in view of heat resistance, environmental pollution and mechanical strength.
There has recently been proposed the art of remedying the drawbacks of the conventional electrophotographic photoreceptor by using amorphous silicon for the photoconductive layer. The a-Si prepared through vapor deposition or sputtering is undesirable for use in such an electrophotographic photoreceptor because its dark resistivity is as low as 10.sup.5 .OMEGA.cm while its photoconductivity is extremely small. In such a-Si, there are formed the so-called dangling bonds with the severed Si-Si bonds and, due to the defect, a number of localized states are present in the energy gap. For this reason, the hopping conduction of the thermally excited carriers occurs and causes the dark resistivity to reduce and further, because the photo generated carriers are trapped by the localized states, the photoconductivity is badly affected.
On the other hand, the aforesaid defect is trapped by hydrogen atoms (H) and Si is thereby bonded to H in hydrogenated amorphos silicon (a-Si(H)) prepared through the glow discharge decomposition of silane gas (SiH.sub.4) or photo CVD, whereby the photoconductivity is improved and p- and n-type valence electron control can readily be conducted because of the far reduced number of dangling bonds. Notwithstanding, its dark resistivity is within the range of 10.sup.8 -10.sup.9 .OMEGA.cm, which is still lower than 10.sup.12 .OMEGA.cm being deemed satisfactory for the photographic photoreceptor. The photoreceptor thus formed of a-Si(H) consequently provides the high dark decay rate of the surface potential and low initial charged potential. However, the resistivity may be increased up to over 10.sup.12 .OMEGA.cm to provide high charge acceptance by doping the a-Si(H) with a proper amount of boron so that it may be applicable to the copying process of the Carlson method.
The photoreceptor with the a-Si(H) as a surface layer allows the acquisition of good images initially but apparently produces not only images of inferior quality very often after it is exposed to the air or stored in a humid environment for a long period of time but also those having a blur gradually after it undergoes a copying process a number of times. Such a degraded photoreceptor tends to produce the blur particularly in a humid environment and, as the number of copying times increases, it has been confirmed that the critical humidity at which the image begins to blur also tends to lower.
As aforementioned, because of the exposure to the air and humidity for a long time, or because of chemical species (ozone, nitrogen oxide, nascent oxygen, etc.) generated by the corona discharge during the copying process, it is considered that the surface of the photoreceptor is easily affected thereby and the change of chemical properties produces inferior images. However, the mechanism of such surface deterioration has not yet been fully examined and attempts have been made to prevent the production of such inferior images and improve printing durability by providing a protective layer on the surface of the a-Si(H) photoreceptor to stabilize its chemical properties. For instance, there is a known method of preventing the surface layer of a photoreceptor from deteriorating because of the copying process or environmental atmosphere by employing hydrogenated amorphous silicon carbide (a-Si.sub.x C.sub.1-x (H), 0&lt;x&lt;1) or hydrogenated amorphous silicon nitride (a-Si.sub.x N.sub.1-x (H), 0&lt;x&lt;1) for the surface protective layer (Japanese Patent Laid Open No. 115559/82).
Although the printing durability is improvable provided the carbon or nitrogen concentration in the surface protective layer is properly selected, a buffer layer must be provided to moderate the material heterogeneity between the a-Si(H) and a-Si.sub.1-x C.sub.x (H), a-Si.sub.1-x N.sub.x (H). As it is preferred to gradually change the bond length and the energy gap in the buffer layer, the mixture ratio of a gas containing Si to what contains C or N has gradually been changed for the purpose. However, the aforesaid process is unfavorably complicated.
As a known buffer layer between the photoconductive layer of amorphous silicon and the surface layer of amorphous carbon, there are a-Si.sub.1-x C.sub.x (H) (0&lt;x&lt;1) and a-Si.sub.1-x C.sub.x (H, F) (0&lt;x&lt;1) (Japanese Patent Application No. 61164/85). Since a mixed gas containing silicon (e.g., SiH.sub.4, Si.sub.2 H.sub.6,SiF.sub.4, etc.) and carbon (e.g., CH.sub.4, C.sub.2 H.sub.6, C.sub.2 H.sub.4, C.sub.2 H.sub.2, C.sub.6 H.sub.6, etc.) is employed to form such a buffer layer as raw materials, the process becomes complicated and a number of checking items are required to realize the value x. In other words, a single kind of gas should preferably be used.